Re: DAX 2MB mappings for XFS

["Kani, Toshi" <toshi.kani@hpe.com>]

On Sat, 2018-01-13 at 08:19 +1100, Dave Chinner wrote:
 :
> IOWs, what you are seeing is trying to do a very large allocation on
> a very small (8GB) XFS filesystem.  It's rare someone asks to
> allocate >25% of the filesystem space in one allocation, so it's not
> surprising it triggers ENOSPC-like algorithms because it doesn't fit
> into a single AG....
> 
> We can probably look to optimise this, but I'm not sure if we can
> easily differentiate this case (i.e. allocation request larger than
> continguous free space) from the same situation near ENOSPC when we
> really do have to trim to fit...
> 
> Remember: stripe unit allocation alignment is a hint in XFS that we
> can and do ignore when necessary - it's not a binding rule.

Thanks for the clarification!  Can XFS allocate smaller extents so that
each extent will fit to an AG?  ext4 creates multiple smaller extents
for the same request.

-Toshi